Platelet Aggregation Dysfunction in Sickle Cell Disease

Acute systemic painful vaso-occlusive crisis (VOC) is the predominant clinical event requiring sickle cell disease (SCD) patients to seek emergency medical care. VOC often serves as an antecedent to acute chest syndrome (ACS), which is a type of acute lung injury and the leading cause of mortality in SCD. Current treatments for ACS are primarily supportive, and there is a critical need for rescue strategies to prevent the progression of VOC to ACS. ACS involves thrombosis across pulmonary artery branches, but the pathophysiology of thrombosis in SCD is largely unknown. Adenosine diphosphate (ADP) released from lysed erythrocytes can activate platelets by stimulating the purinergic receptors P₂Y₁ and P₂Y₁₂, suggesting that blocking these receptors may prevent VOC. Surprisingly, P₂Y₁₂ receptor blockade was unsuccessful in preventing VOC in a recent DOVE trial, justifying the need for better understanding of purinergic signaling in SCD.

We used a novel intravital microscopy enabled experimental model of vaso-occlusive pulmonary thrombosis in transgenic SCD mice triggered by an intravenous (IV) administration of ADP. This method allows real time in vivo visualization of platelet aggregation in the lung of live SCD mice using multi-photon-excitation based quantitative fluorescence intravital lung microscopy (qFILM). We discovered that vaso-occlusive thrombosis involves entrapment of large circulating platelet aggregates in the bottle-necks located between the pulmonary arterioles and capillaries. Surprisingly, the dose of ADP that led to the pulmonary vaso-occlusive thrombosis in control mice, failed to induce vaso-occlusive thrombosis in SCD mice. Remarkably, further in vivo and in vitro platelet aggregation studies confirmed that SCD mouse platelets were indeed refractory to ADP but not collagen.

Our preliminary findings suggest that therapeutic inhibition of P₂Y₁ and P₂Y₁₂ platelet receptors will not be a successful therapy in SCD. Our findings also warrant the need for further studies to understand the role of purinergic signaling in SCD